1. Field of the Invention
The present invention relates to a dynamic optical fiber, sensor of physical quantities, that is to say, one that is insensitive to static quantities, notably to the static quantities relating to the physical quantity to be measured.
2. Description of Related Art
One possible application of such sensors is a fiber-optic hydrophone. The most efficient existing systems rely on the use of distributed feedback fiber lasers, and they are used in complex architectures that implement interferometric interrogation devices.
The two main defects are:                the high sensitivity of these sensors to static pressure and to temperature,        the complexity of the interrogation systems.        
The optical fiber-based sensors have been researched for close on thirty years. These sensors benefit from the advantages of optical fibers which, in addition to their low weights, bulk, cost and insensitivity to electromagnetic disturbances, exhibit low losses, a high bandwidth and are suited to the multiplexing techniques and to the implementation of amplifiers or distributed sensors.
The applications of optical fiber sensors are wide-ranging. The commonest relate to the detection of stresses, of temperature and of pressure, but they also exist as current/voltage, magnetic field, displacement, torsion, acceleration, gas and other detectors. The techniques used are also very varied, the most actively researched being fiber gyroscopes, other interferometric methods, and back scattering (Raman, Brillouin or Rayleigh) techniques. Almost half of the fiber sensors currently being researched implement Bragg gratings. In particular, the use of active sensors (lasers) based on Bragg gratings is becoming widespread: these are DBR (Distributed Bragg Reflector) lasers or DFB (Distributed FeedBack) lasers, the spectral purity of which provides for a high gain in terms of sensitivity compared to devices with passive Bragg gratings.
In the case of hydrophones with Bragg grating fiber, the quantity actually measured by the system is the axial deformation of the element with Bragg grating fiber induced by the pressure wave. For applications relating to hydrophones for submarine detection, the low pressure level that is to be detected (“sea 0” noise level according to the Knudsen scale) generally requires a mechanical device around this fiber element that is designed to amplify the transfer function between the external pressure and the axial deformation of the fiber. The order of magnitude of the deformations to be measured in this case is situated between 10−9 (nanostrain) and 10−12 (picostrain). The deformation on the sensor induces a phase shift on the optical wave that is propagated therein, which is reflected in the case of an active cavity with Bragg grating (DFB or DBR laser) in a variation of the optical frequency of the laser. Measuring this phase shift or this frequency variation entails comparing the frequency of the wanted signal with that of a reference signal. Among the methods used, there are primarily two solutions for obtaining a reference wave. The first solution (heterodyne type) consists in using a reference wave originating from a third-party sensor, similar but isolated from the disturbance. The second solution (“self-homodyne” type) consists in splitting the wanted signal into two arms with very different optical paths and in producing interferences between these two arms. In this case, the reference wave is a delayed copy of the signal wave.
One of the limitations of the current fiber laser hydrophones is the influence of the static pressure on the laser operation: under the pressure of the water either the cavities no longer emit, or their emission wavelengths are modified to the point of corrupting the operation of the system. In practice, the pressure of the water increases by approximately 1 bar every 10 m. However, these systems are intended for deep-immersion uses, that is to say, at depths of the order of 100 to 800 m. The static pressure modifies the length of the laser cavity and provokes a translation of the emission wavelength that is all the greater as the sensor is designed to be sensitive to very low dynamic pressure levels (approximately 3 nm to 400 m of depth in the case of a hydrophone optimized for submarine detection). In the case of wavelength multiplexed architectures, for example, the static pressure is a direct limitation of the spacing between two wavelengths, and consequently reduces the maximum number of sensors that can be arranged in series on a single fiber. There are solutions available for overcoming this problem. It is possible either to measure the static pressure, then take account of it when processing the data, or to compensate for it. The first method is expensive and limits the sensitivity of the system. The second method requires sophisticated mechanical and/or piezoelectric devices to filter, at the level of the mechanical deformation amplification device, the very low frequency portion.